Systems for delivering laser energy onto tissue are well known in the art. The use of laser energy provides a less invasive style of surgical intervention. This style is characterized by achieving a maximal therapeutic effect with minimal damage to surrounding and overlying normal tissues.
For performing precision surgical procedures, the laser energy can be transmitted through an optical fiber introduced via endoscopic devices which pass through the body's natural orifices, or through optical fibers inserted transdermally in minimally invasive procedures.
Wavelengths that can be effectively transmitted through an optical fiber are dependent on the propagation loss and the dispersion loss of the light as it propagates through the fiber. Therefore, the spectral bandwidth of the laser energy plays an important role in the dispersion-limited propagation, while the laser input power plays a crucial role in the absorption-limited regime.
Merely increasing the level of input power through an optical fiber, even below obvious limits of material damage, will not necessarily result in a higher output power. Nonlinear interactions such as scattering and the affects of heat due to over pumping become important at high laser intensities. Thus, transmission at both a wavelength and power level compatible with the optical fiber is the most effective way of preventing optical loss and delivering laser energy. Currently, optical energy having a wavelength in the range of less than about 3 .mu.m can effectively travel through conventional optical fibers.
The effect of laser energy on tissue is a function of laser wavelength, energy density, time of exposure, and tissue absorption. Changing these parameters will result in a variation in the amount of heat generated in the tissue. The interaction between temperature and the affected tissue area determines whether the tissue will be cut, vaporized or coagulated by the laser energy.
As known by those skilled in the art, biological tissue comprises cells embedded in a primarily proteinaceous extracellular matrix. Collagen is one of the predominate proteins found in the extracellular matrix. Collagen can be altered by the application of energy, such as provided by a laser. At relatively low energy, collagen can be cross linked, reducing its volume and increasing shrinkage or tightening of tissue. At higher energies, collagen can be denatured and form a biological glue. For such a subcutaneous surgical procedure, it is desirable to provide pulsed laser energy at a frequency of about 20 to 80 Hertz (Hz) with each pulse providing energy in the range of about 0.1 to 30 milliJoules (mJ) or less, and preferably about 1 to 10 mJ.
As indicated previously, manipulation of the amount of laser power is a practical expedient during a surgical procedure when laser energy is transmitted through an optic fiber. The wavelength range which can be transmitted through a standard optic fiber is substantially limited however.
Two laser sources which are particularly useful in providing laser energy at a wavelength that is transmittable through an optical fiber are excimer lasers and near infra red lasers. The term excimer describes a family of lasers which emit powerful pulses lasting nanoseconds or tens of nanoseconds at wavelengths in or near the ultraviolet. Near infrared lasers are typically solid state lasers in which the active medium is a nonconductive solid, a crystalline material, or glass doped with a species that can emit laser light.
Excimer lasers and many types of solid state lasers can operate only in a pulsed mode for a variety of reasons due to internal physics. One of the most widely used medical solid state lasers, holmium, can operate in the quasi-continuous wave at cryogenic temperatures, but operates in the pulsed mode at room temperatures.
Typically, it is desirable to change the "natural" duration of the laser pulses widths in order to control the pulse energy density delivery. Mostly, the goal is to shorten each energy pulse to provide higher instantaneous peak power. Normally, the pulse repetition rate, pulse rate duration, and peak power can be adjusted by a variety of methods such as intercavity Q-switching which functions by eliminating the gain feedback inside a laser resonator, in essence storing energy in the population inversion, to produce a pulse that is shorter in width and higher in peak power than an unaltered laser emission. Correspondingly, changing the pulse width gives the laser user added flexibility in matching laser characteristics to satisfy certain operational requirements (i.e., clinical application, tissue integration, etc.).
Although Q-switching performs well with many laser types, intercavity Q-switching of excimer lasers is impractical because of the high internal gain of such lasers. Likewise, intercavity Q-switching of holmium lasers is ineffective because of the incomplete energy transfer which can result in a large pulse-to-pulse variation in pulse amplitude. Additionally, the high peak intercavity fluences can cause coating damage to the rod and the optics surface adding to the ineffectiveness and impracticality. Thus, the current methods fail to provide for precisely controlling the duration of time and, hence the amount of laser energy being emitted onto tissue.
The present invention provides a device which overcomes the above problems by selectively attenuating a laser pulse after the pulse has been emitted from a laser resonator to control the amount of energy density delivered onto tissue.